It is known that a bulk acoustic-wave (BAW) device is, in general, comprised of a piezoelectric layer sandwiched between two electronically conductive layers that serve as electrodes. When a radio frequency (RF) signal is applied across the device, it produces a mechanical wave in the piezoelectric layer. The fundamental resonance occurs when the wavelength of the mechanical wave is about twice the thickness of the piezoelectric layer. When fabricating BAW devices by depositing thin-film layers on passive substrate materials, one can extend the resonance frequency to the 0.5–10 GHz range. These types of BAW devices are commonly referred to as thin-film bulk acoustic resonators or FBARs. FBARs are usually used in combination to produce passband or stopband filters. The combination of one series FBAR and one parallel, or shunt, FBAR makes up one section, or stage, of the so-called ladder filter. The description of ladder filters can be found, for example, in Ella (U.S. Pat. No. 6,081,171). As shown in FIG. 8f in Ella, two series-connected BAW resonators (hereafter referred to as series resonators) and two shunt-connected BAW resonators (hereafter referred to as shunt resonators) are used to form a two-stage passband filter in a ladder circuit or topology. BAW ladder filters are typically designed so that the series resonators yield a series resonance at a frequency that is approximately equal to, or near, the desired center frequency of the filters, and that the shunt resonators yield a shunt resonance at a frequency that is approximately equal to, or near, the desired center frequency of the filters.
In a typical telecommunication device, such as a mobile phone, an antenna is coupled to both the input of a receiver and to the output of a transmitter. In such an arrangement, a duplexer is used to provide the necessary coupling between the antenna and the respective transceiver. The duplexer is also used to prevent the modulated transmitted signal from being coupled to the receiver. A duplexer is one of key components in a full duplex cellular phone. In the CDMA (code division multiple access) and WCDMA (wideband CDMA) standards, a duplexer is required. In prior art, lattice type filters using surface acoustic wave (SAW) or BAW resonators have been incorporated into a duplexer. Bradley et al. (EP 1 058 383 A2, hereafter referred to as Bradley) discloses a duplexer formed from a plurality of BAW resonators. One of the embodiments of the FBAR duplexer in Bradley is shown in FIG. 1. As shown in FIG. 1, the duplexer 900 comprises a first passband filter 930 connected between the first port 924 and the second port 928, and a second passband filter 932 connected between the second port 928 and the third port 926 via a 90° phase shifter 934, which is used for matching the first passband filter 930 and the second passband filter 932 at the second port 928. Both passband filters 930, 932 are FBAR ladder-type devices. The first passband filter 930 is a 2½-stage ladder circuit, formed by three series resonators 901, 903, 905 and two shunt resonators 907, 909. The second passband filter 930 is a 3½-stage ladder circuit, formed by three series resonators 911, 913, 915 and four shunt resonators 921, 923, 925, 927. A stage, regarding a ladder circuit, is a basic construction cell consisting of one series resonator and one shunt resonator.
Current front-end designs of cellular phones are singled-ended. There is progress towards a fully balanced front-end. A balanced filter provides better electrical performance than a single-ended filter. In a balanced filter, the insertion loss is typically less than a ladder-type filter, and the stopband attenuation is much higher than in a ladder-type filter. Typically, stopband antennuation of a ladder filter is 25 dB. While the electrical performance of a ladder-type filter and duplexer is acceptable, it is desirable and advantageous to provide a passband filter and duplexer with improved electrical performance.
Furthermore, in a telecommunication device where two communication channels use two closely spaced frequencies, it is advantageous and desirable to provide a dual-channel for transmitting and receiving signals in those frequencies.